1. Field of the Invention
The present invention relates to a sensorless start-up method for driving a brushless DC motor.
2. Description of the Related Art
Along with the advancement of technology, motors play a more and more important role in everybody's daily life, such as spindle motors of disc players, drive motors of image scanners, motors in toys, and motors of windscreen wipers. Due to the lack of commutating brushes, permanent magnet brushless DC motors is easier to be taken care of and has a smaller size, a higher efficiency, and other advantages, all of which render the brushless DC motors widely applied in many areas.
A conventional method for driving the permanent magnet brushless DC motor is to detect the position and rotating rate of a rotor through the use of Hall sensors in order to effectively perform the control on the position and rotating rate. However, the Hall sensors tend to be influenced by the operational environment, which adversely reduces the accuracy of detection. For this reason, a sensorless driving method is introduced into the mainstream of technological development. With respect to the sensorless driving method, the position of the rotor is detected on the basis of BEMF (back electromotive force). However, BEMF becomes almost zero when the rotor is static or at a very low rate of rotation, resulting in the impossibility of detecting the rotor's position. Therefore, one of the most important issues is how to accurately and stably start driving the motor by the sensorless method.
Among the proposed start-up methods, the open-loop control is the most widely employed. Three tri-phase sinusoidal waves, each of which has an increasing frequency, are directly applied to three phase coils of the motor so as to raise the rate of rotation of the motor. However, the start-up process may fail since the largest torque may not be generated upon the start-up due to the lack of knowledge about the rotor's position. In order to avoid such failure of start-up, the stator's coils may at first be energized for aligning the rotor in a predetermined direction, and then the open-loop control is executed. Nonetheless, the energization of the stator's coils may cause the rotor to reversely rotate, causing limitations to the applicable range. Moreover, the third method is to firstly detect the initial position of the rotor, and then starts driving the motor by the open-loop control in accordance with the rotor's initial position, thereby avoiding the failure of the start-up and reverse rotation.
Many techniques have been proposed to detect the initial position of the rotor in accordance with a functional relationship between the coil inductance and the rotor's position. Unfortunately, these techniques require either complicated algorithms or the feedback of tri-phase voltages and currents. In U.S. Pat. No. 6,946,808, entitled “Motor Drive Control Circuit And Motor Drive Apparatus,” the rotor's initial position is detected by way of firstly applying a set of detection signals and then determining the rotor's initial position in accordance with responses of a common-terminal voltage of the stator's coils. Such a technique has an advantage of being easily carried out by using analog circuitry. However, it is difficult to retrieve the common-terminal voltage because the common terminal of the stator's coils is not qualified as a standard input/output interface of the motor structure.